Re: [PATCH bpf-next v2 18/18] docs/bpf: Update documentation for BTF_KIND_ENUM64 support

[Yonghong Song <yhs@fb.com>]

On 5/17/22 4:47 PM, Andrii Nakryiko wrote:
> On Fri, May 13, 2022 at 8:14 PM Yonghong Song <yhs@fb.com> wrote:
>>
>> Add BTF_KIND_ENUM64 documentation in btf.rst.
>> Also fixed a typo for section number for BTF_KIND_TYPE_TAG
>> from 2.2.17 to 2.2.18.
>>
>> Signed-off-by: Yonghong Song <yhs@fb.com>
>> ---
> 
> LGTM, but see pedantic note below
> 
> Acked-by: Andrii Nakryiko <andrii@kernel.org>
> 
>>   Documentation/bpf/btf.rst | 34 +++++++++++++++++++++++++++++-----
>>   1 file changed, 29 insertions(+), 5 deletions(-)
>>
> 
> [...]
> 
>> +2.2.19 BTF_KIND_ENUM64
>> +~~~~~~~~~~~~~~~~~~~~~~
>> +
>> +``struct btf_type`` encoding requirement:
>> +  * ``name_off``: 0 or offset to a valid C identifier
>> +  * ``info.kind_flag``: 0 for unsigned, 1 for signed
>> +  * ``info.kind``: BTF_KIND_ENUM64
>> +  * ``info.vlen``: number of enum values
>> +  * ``size``: 1/2/4/8
>> +
>> +``btf_type`` is followed by ``info.vlen`` number of ``struct btf_enum64``.::
>> +
>> +    struct btf_enum64 {
>> +        __u32   name_off;
>> +        __u32   val_lo32;
>> +        __u32   val_hi32;
>> +    };
>> +
>> +The ``btf_enum64`` encoding:
>> +  * ``name_off``: offset to a valid C identifier
>> +  * ``val_lo32``: lower 32-bit value for a 64-bit value
>> +  * ``val_hi32``: high 32-bit value for a 64-bit value
>> +
> 
> I presume if size is < 8 then val_hi32 will be sign-extended (i.e.,
> 0xffffffff for signed enum and negative enumerator values, 0
> otherwise), right? Should it be specified here?

Right. This is similar to btf_enum, e.g., btf_enum can have size 1,
e.g.
   enum T { A = -1, B, C, D } __attribute__((packed));
But for enumerator A, its value will be 0xffffffff.

Here, val_lo32 and val_hi32 are just low and high 32bits
of unsigned long long value of the enumerator. I will add
a few sentences to clarify here.

> 
>>   3. BTF Kernel API
>>   =================
>>
>> --
>> 2.30.2
>>